Removed the log10 declaration in RealFieldElement interface.

This method introduced a binary incompatibility as one implementing
class (Dfp) did not have the proper return type and had to be changed.
The method will be added back in the interface for 4.0, when it will be
allowed to change the return type for Dfp.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/math/trunk@1455053 13f79535-47bb-0310-9956-ffa450edef68